WELDING AND CUTTING TECHNIQUES

Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material (the weld pool) that cools to become a strong joint, with sometimes used in conjunction with heat, or by itself, to produce the weld.

coalescence -  When two, or more, pieces of metal are bonded together (usually via welding) by liquefying the places where they are to be bonded, coalescing these liquids, and allowing the coalesced liquid to solidify. At the end of this process the two pieces of metal have become one continuous solid.

WELDING POWER SUPPLY - Constant current supplies are used in manual welding processes as it can be difficult to hold the electrode perfectly steady, and as a result, the arc length and thus voltage tend to fluctuate. Constant voltage supplies are used in automated processes as the arc length is maintained constant easily.

     Welding power supplies makes uses of 

• AC transformers (Step Down), AC to DC rectifiers or
• Inverter controlled power supplies
•  DC generators.

    Welding power supplies making use of AC transformers (Step down) and rectifiers (AC to DC) would have an input power supply of 60 HZ, 220V or 460V or 575V which is high voltage and less current, which will be fed to a step-down transformer convert it to a 60 Hz less voltage and higher current and thereafter rectified From AC to DC if necessary.

     In inverter controlled power supplies, the same incoming 60 Hertz power is used. However, instead of being fed directly into a transformer, it is first rectified to 60 Hertz DC. Then it is fed into the inverter section of the power supply where it is switched on and off by solid state switches at frequencies as high as 20,000 Hertz. This pulsed, high voltage , high frequency DC is then fed to the main power transformer, where it is transformed into low voltage 20,000 Hertz DC suitable for welding. Finally it is put through a filtering and rectifying circuit . 

      Welding power supplies may also use generators or alternators to convert mechanical energy into electrical energy. Modern designs are usually driven by an internal combustion engine but some older machines use an electric motor to drive the alternator or generator. In this configuration the utility power is converted first into mechanical energy then back into electrical energy to achieve the step-down effect similar to a transformer. Because the output of the generator can be direct current, these older machines can produce DC from AC without any need for rectifiers of any type.

   ELECTRODE SELECTION - Based on the type of welding, electrode selection is limited to be either consumable electrode or non-consumable electrode.

  Welding processes like Shielded Metal Arc Welding (SMAW) and Gas Metal Arc Welding (GMAW), Explained Later make use of consumable electrodes and welding processes like Gas Tungsten Arc Welding and Oxy Fuel welding uses Non-Consumable Electrodes with Separate Filler material.

      In General Welding electrodes are specified as combination of 

   ALPHABET NUMBER NUMBER NUMBER NUMBER  like E7018.

in the above combination, 

                          E indicates that the electrode is for Electric arc welding.

                         First two digits,70 indicates the tensile strength of the electrode metal in thousands of pounds per square inch. here it is 70*1000 = 70000 PSI.

                          Third Digit, 1 indicates electrode is suitable for all welding positions (welding positions explained later).

                          Fourth Digit, 8 indicates the polarity (polarities explained later) for the electrode either positive or negative. In this case its 8 which represents AC or DC+. 

      here AC means when AC power supply is used it doesn't matter with which polarity you connect the electrode but when DC power supply is used, electrode should be connected to the reverse polarity that is to the positive side of the supply.

                          Combination of Third and Fourth Digits represent the flux coating. here it 18.

    There may be times when a suffix is added to the code on rod. This indicates the the type of alloy or material that was used on the coating.

      Practically, type of electrode depends on the base metal being welded and used current depends on the thickness of the base metal.

      Usually we join similar metals and so the electrode selected would be the same as the base metals that are need to be joined. 
       As the diameter of the selected welding electrode increases, higher currents are required to melt it. 

      The most common metal you will ever weld on and to will be carbon steel. Carbon steel also known as iron, is the most basic metal and probably the most common. It is an element from natural deposits that has been smelted or refined into structural or pipe steel. 

      Electrode manufacturers specify the type of power source to be used with the electrode like AC or DCRP (Direct current reverse polarity) , DCSP (Direct current straight polarity). electrodes should be used as specified. As we know the last digit of the electrode number represents the polarity. 

       When welding to steel or iron the most common rods will be carbon based rods.

E6010,E7010, maybe E8010. These ten series AC rods are fast freeze rods that are basically the same in their characteristics and function. DC rods that are available like E6011 and E6013. These rods are good for fusing basic iron components together. Farm equipment, structural steel, pipe welding, shipyards, shop work, etc.

       The complimentary rods that can go with these in similar applications would be 7018, 8018, all the way up to 11018. 70,000 to 110,OOOlb. tensile strength. These rods are not fast freeze AC rods they produce less sparks and spatter while welding. They are slower moving rods that produce a more consistent puddle and have more ductility than fast freeze rods.

       Fast freeze rods tend to be more brittle in composition. The difference is mainly in the flux.

Ductility is the ability of metal to be drawn out into wire. This means that in this process the metal doesn't break or fatigue. There is a resilience in the metal to maintain its core characteristics.

There are special rods for cast iron welding. Metals that are cast have a crystalline core that is tough to weld on. Of course there are many others. The cast iron process is actually best welded with brass. Brass is very difficult to make nice. Some welding is good looking but brass is not.

Shipyards use brass on propellers and props. Brazing is a popular method of welding with brass. There are rods that are more like a tig filler metal that have a flux coating around them that are used with a torch burning process and not electricity.

Inkonel is an iron/nickel composite for cast iron. Cast iron welding requires massive heat applied to the metals and a slow cool down period to keep the weld from cracking.

      Now i like to explain how we select the electrode in my job site. 

     We use E6013 electrode which is 3.15mm in diameter for pipe works, sheet metal works. Depending on the thickness of th base metal current is adjusted accordingly. if wrong current is selected, for example if higher currents are selected when welding with E6013 on a 3 mm thick sheet metal, it makes hole to the base metal because higher current produces higher heat output in such a way that base metal cannot take the heat but melts. If the thickness of the base metal increases current needs to be increased.

    We also use E7018 electrodes for the jobs where strength is more important like welding a lifting eye which holds 10 tons of Load. In this case welding is more important because if the welding fails it just relieves the 10 ton load to earth which is not safe. Assume that we have selected the same current when using E7018 as selected for E6013 (assume electrode diameter is same). That current will not melt the rod sufficiently to weld. It melts a little bit and then sticks to the base metal when u try to move the electrode. This is because E7018`s is a stronger rod whose tensile strength is higher than that of E6013 and requires more heat to get melted. So current needs to be increased.

   We also use Hard facing electrodes where wear resistant surfaces are needed. Hard facing electrodes have higher percentage of Chromium and nickel when compared to normal electrodes and so demand higher currents. Welding is done as several passes on the surface which is subjected to wear.

   The best example is the picture shown below - hard facing on excavator bucket.

    Thinking how to increase and decrease the current?

on the welding machine there is a selection rotation switch like the volume control on a stereo. moving it anti clockwise will decrease the current and anti-clockwise will increase the current.

When talking about WELDING several types of welding comes in picture.

         The oldest welding technique ever known was FORGE WELDING.

         Some of the most common welding processes we see in our daily life are SHIELDED ARC WELDING AND OXY-FUEL WELDING (GAS WELDING).

         The Industrial Welding Techniques are listed below.

1. ARC WELDING 
• Shielded metal arc welding (SMAW) or stick welding
•  Gas metal arc welding (GMAW)or MIG welding
•  Flux-cored arc welding (FCAW)
• Gas tungsten arc welding (GTAW) or TIG welding
•  Submerged arc welding (SAW)        
2. OXY FUEL WELDING AND CUTTING
3. ELECTRIC RESISTANCE WELDING
   • Spot Welding
   • Seam Welding
4. ELECTRO-SLAG WELDING
5. LASER BEAM WELDING
6. ROBOT WELDING

 Lets go into the details .....

FORGE WELDING - is a solid-state welding process that joins two pieces of metal by heating them to a high temperature and then hammering them together.The process is one of the simplest methods of joining metals and has been used since ancient times. Forge welding is versatile, being able to join a host of similar and dissimilar metals.

Forge welding between similar materials is caused by solid-state diffusion. This results in a weld that consists of only the welded materials without any fillers or bridging materials.

 Forge welding between dissimilar materials is caused by the formation of a lower melting temperature eutectic between the materials. Due to this the weld is often stronger than the individual metals.

       ARC WELDING - Principle - An electric arc is created and maintained between the electrode and the base metal using power supply which melts the metals at the welding point. 

            It makes use of either AC or DC power supply i.e., constant current or constant voltage and either consuming or Non-consuming Electrodes. The welding region is sometimes protected by some type of inert or semi-inert gas, known as a shielding gas, and filler material is sometimes used as well.
                
 details for types of Arc Welding...

     
          SHIELDED METAL ARC WELDING - is also known as manual metal arc welding (MMA) or stick welding. Electric current is used to strike an arc between the base material and consumable electrode rod, which is made of steel and is covered with a flux that protects the weld area from oxidation and contamination by producing carbon dioxide(CO₂) gas during the welding process. The electrode core itself acts as filler material, making a separate filler unnecessary.

Practical Explanation:  In the following picture you can see typical SMAW welding equipment as a whole. 


   SMAW requires a welding power source, two cables - one for electrode holder and the other for Earth, a face shield for protecting your eyes from light and fumes, gloves for preventing the metal sparks burning your hands, Consumable Electrode as per requirement, chipper and cleaner to clean the slag.


   

     Typical welding equipment for SMAW is shown below.

       When using power supplies with AC output, the electrode holder is connected to positive terminal and the negative terminal or ground terminal is connected to the workpiece to close the circuit.
        When using power supplies with DC output, the connection terminal for the electrode (same as the electrode holder) depends on the type of electrode used. (See electrode selection)

         The electrode is placed in the electrode holder and when touched by the workpiece or base metal an arc is produced as per the set current output. 

STARTING THE ARC 
Two basic methods are used for starting the arc: the STRIKING or BRUSHING method and the TAPPING method.  In either method, the arc is started by short circuiting the welding current between the electrode and the work surface. The surge of high current causes the end of the electrode and a small spot on the base metal beneath the electrode to melt instantly.

In the STRIKING or BRUSHING method, the electrode is brought down to the work with a lateral motion similar to striking a match. As soon as the electrode touches the work surface, it must be raised to establish the arc. The arc length or gap between the end of the electrode and the work should be equal to the diameter of the electrode. When the proper arc length is obtained, it produces a sharp, crackling sound.

In the TAPPING method, you hold the electrode in a vertical position to the surface of the work. The arc is started by tapping or bouncing it on the work surface and then raising it to a distance equal to the diameter of the electrode. When the proper length of arc is established, a sharp, crackling sound is heard.

When the electrode is withdrawn too slowly with either of the starting methods described above, it will stick or freeze to the plate or base metal. If this occurs, you can usually free the electrode by a quick sideways wrist motion to snap the end of the electrode from the plate. If this method fails, immediately release the electrode from the holder or shutoff the welding machine. Use a light blow with a chipping hammer or a chisel to free the electrode from the base metal.

An arc is not produced only if the circuit is not closed. A low arc is produced only if the set current does not meet the requirement of the used electrode i.e., the set current is not sufficient to produce enough heat to melt the electrode.
         
               Once the set current produces enough arc to melt the electrode, welding will be in progress. 
    When the current is too high, the electrode melts faster and the molten puddle will be excessively large and irregular. High current also leaves a groove in the base metal along both sides of the weld. This is called undercutting view C in the figure below.

    With current that is too low, there is not enough heat to melt the base metal and the molten pool will be too small. The result is poor fusion and a irregular shaped deposit that piles up. View B in the figure Above. 

    This piling up of molten metal is called overlap. The molten metal from the electrode lays on the work without penetrating the base metal. Both undercutting and overlapping result in poor welds, as shown in figure below.

       As the consumable electrodes have limited lengths normally 350mm, 400mm and 450 mm the electrode should be frequently changed. Whenever fresh electrode is used the protective layer or slag should be chipped out.

        Arc length is an important criteria while welding. The arc length should be maintained nearly constant to prevent improper welding.  

When an arc is too long, the metal melts off the electrode in large globules and the arc may break fre-quently. This produces a wide, spattered, and irregular deposit with insufficient fusion between the base metal and the weld.

When an arc is too short, it fails to generate enough heat to melt the base metal properly, causes the electrode to stick frequently to the base metal, and produces uneven deposits with irregular ripples. The recommended length of the arc is equal to the diameter of the bare end of the  electrode.

Advantages    

1.  Shielded manual metal arc welding is the simplest of all the welding processes.
2.  The equipment can be portable and the cost is fairly low.
3.  This process finds innumerable applications, because of the availability of a wide variety of electrodes.
4.  A big range of metals and their alloys can be welded.
5.  Welding can be carried out in any position with highest weld quality.
6.  The process can be very well employed for hard facing and metal deposition to reclaim parts or to develop other characteristics like wear resistance etc.
7. Joints (e.g. between nozzles and shell in a pressure vessel) which because of their position are difficult to be welded by automatic welding machines are easily accomplished by flux shielded metal arc welding.

Disadvantages

1. Because of the limited length of each electrode and brittle flux coating on it mechanization is difficult.
2. In welding long joints (e.g., in pressure vessels), as one electrode finishes, the weld is to be progressed with the next electrode. Unless properly cared, a defect (like slag inclusion or insufficient penetration) may occur at the place where welding is restarted with the new electrode.
3. The process uses stick electrodes and thus it is slower as compared to MIG welding (MIG welding explained later).
4. Because of flux coated electrodes, the chances of slag entrapment and other related defects are more as compared to MIG or TIG welding (MIG and TIG weldings explained later).
5.  Because of fumes and particles pf slag, the arc and metal transfer is not very clear and thus welding control in this process is a bit difficult as compared to MIG welding.

Applications

1. Today, almost all the commonly employed metals and their alloys can be welded by this process.
2. Shielded metal arc welding is used both as a fabrication process and for maintenance and repair jobs.
3.  The process finds applications in 

• Air receiver, tank, boiler and pressure vessel fabrications;
• Ship building
• Pipes and Penstock joining
• Building and Bridge construction
• Automotive and Aircraft industry, etc.,

      
GAS METAL ARC WELDING - Commonly known as Machine Inert Gas welding (MIG). The principle of arc formation is same as in Arc welding but it is totally different process.

WELDING GEOMETRY - Welds can be geometrically prepared in many different ways. The five basic types of weld joints are the butt joint, lap joint, corner joint, edge joint, and T-joint.

BUTT JOINT - a joint between two members aligned approximately in the same plane

         LAP JOINT - a joint between two overlapping members

         CORNER JOINT - a joint between two members located at right angles to each other

        EDGE JOINT - A joint between the edges of two or more parallel or nearly parallel members.

         T JOINT - a joint between two members located approximately at right angles to each other in the form of a T .